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Screening Weeds for Herbicide Resistance, An Underutilized Resource

Posted by Doug Finkelnburg | August 13, 2021

We have the technology… so let us quit spraying and praying!

“Do you have herbicide-resistant weeds in your fields?” This is a question I have asked at every grower’s meeting I have spoken at recently. Usually, I get a room (or farm shop) full of blank faces as a reply. Sometimes a brave soul will speak up and say something to the effect of, “Well, I assume I have some out there.” They are probably correct. It is well established that within the inland PNW we have developed herbicide-resistant cheatgrass (a.k.a downy brome), Italian ryegrass, prickly lettuce, mayweed chamomile, and the list goes on. However, just because a weed has been shown to develop resistance to an herbicide, they are obviously not all resistant to it in all places.

How do we know where the resistant weeds are? This seems easy enough to answer, “The ones that didn’t die are resistant” could be a pithy reply except weather, crop stage, weed stress, and mixing/loading/application errors can all contribute to a less than successful application. If you have targeted weeds left alive after an herbicide application, how do you know if they are resistant to the product’s active ingredient? The University line to date has largely been to suspect resistance if one can rule out the above-mentioned complicating factors, especially when one species (or a significant patch of one species) is left alive. A classic example would be the glyphosate resistant kochia pictured in a sugar-beet field. This is good advice, but I will be the first to point out it is not always practicable advice considering the time and attention it takes to do the sleuthing necessary to be certain.

Herbicide resistant kochia in sugar beets. Photo credit: Joel Felix, OSU Extension.
Fortunately, science and technology have given us another way to determine if weeds on the farm are resistant to specific herbicides. University of Idaho, Oregon State, and Washington State University all have weed screening programs for herbicide resistance. Some growers are taking advantage of these labs but they are a small minority. It works like this, you collect weed seeds from plants that you suspect may have resistance to one or more herbicides, put them in an envelope, provide a little contextual information and mail them to a weed science program. They raise the plants in a greenhouse and test them for resistance. Sometime later you have specific information about your weed population’s resistance or susceptibility. Then you can draw up an herbicide program to specifically target to your situation.

The specific knowledge gained from testing weed resistance can return value directly to the farm operation in the form of not spending money on products that will not work or are not necessary. For those renting land with resistant weed issues, testing could give the grower a strong, evidence-based argument to use when negotiating additional herbicide cost-share or even proposing a change in crop rotation or other management practices aimed at addressing the issue. We recognize the importance of soil and tissue testing for fertility management, why on earth are we not widely using the capability to test for herbicide resistance when building weed management programs?

Testing capacity could become an issue since these University labs and programs are not currently set up to test everyone’s farms and fields. I see this as a temporary issue that will be addressed as testing popularity increases. Either the universities will invest in expanded capacity, or some enterprising private business will fill the gap. We now have the technology to significantly aid in building informed, targeted herbicide programs.

Let’s use it.

Let’s quit spraying and praying!

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